The present invention relates to holographic printers. According to a preferred embodiment a method and apparatus for recording and printing holographic stereograms from digital data is disclosed.
For over 50 years holograms have been produced by the general technique of illuminating an object with coherent light and arranging that the scattered light falls onto a photosensitive recording material that is also illuminated by a mutually coherent reference beam (see for instance E. N. Leith et al., “Reconstructed Wavefronts and Communication Theory”, Journal of the Optical Society of America 53, 1377-81 1963). However, with such a technique one requires a physical object in order to make an holographic representation of this object and usually the size of the holographic image corresponds in a 1:1 fashion with the size of the object holographed. For many practical applications this technique is hence unsuitable.
An alternative technique of generating and then directly writing the fundamental interference pattern that characterizes an hologram has been discussed and investigated (see for instance U.S. Pat. No. 4,701,006). However, even with today's computer resources, calculation of the interference pattern by Fourier transforms remains a daunting computational task for larger holograms. In addition it is still highly difficult and costly to write such patterns once calculated, the preferred technique being by electron beam.
Another technique for the generation of holograms that does not require an actual object was proposed by King et al (Applied Optics, 1970). In this paper it was shown that holograms can be composed by optically multiplexing information taken from a plurality of 2-D camera views. The importance of this idea is that the machine that prints the final holograms can be separate from the actual object and that the holographic image does not have to correspond in size to the original object. Further, it has been shown that an object is not required at all if the 2-D views are generated from raw computer data (see for example U.S. Pat. No. 3,843,225).
In a common embodiment of the above principle it is known to record sequential views of an object by a camera mounted on a linear or circular track. Each of the views is then used in an optical system that multiplexes the data together to form an intermediate (or H1) hologram such as described in U.S. Pat. No. 3,832,027. Such a hologram can then be converted or transferred to form a second hologram which is now white-light viewable and is known as the H2 hologram. In order to effect this the H1 hologram is illuminated by laser light in a time-reversed geometry and the real image so produced is used as the object for the H2 hologram. Upon illumination of this H2 hologram by a time-reversed reference beam a white-light viewable virtual image is reconstructed. An efficient and practical commercial machine for converting H1 holograms to H2 holograms is known (see M. V. Grichine, D. B. Ratcliffe, G. R. Skokov, “An Integrated Pulsed-Holography System for Mastering and Transferring onto AGFA or VR-P Emulsions” Proc. SPIE Vol. 3358, p. 203-210, Sixth International Symposium on Display Holography, Tung H. Jeong; Ed.)
Holographic printing techniques which implicitly require the generation of an intermediate, or H1, hologram which is thereafter used to produce a final white-light-viewable hologram are commonly referred to as “2-step” holographic printing processes. Essentially all the major features of known “2-step” holographic printing processes are explained in U.S. Pat. No. 3,832,027. Subsequent developments (e.g. Spierings W. et al., “Development of an Office Holoprinter II”, SPIE Vol. 1667 Practical Holography VI 1992) have replaced the photographic film used in U.S. Pat. No. 3,832,027 with an LCD screen.
An alternative scheme to the “2-step” printing process is described in U.S. Pat. No. 4,206,965 whereby the photographic images are directly multiplexed onto the final white-light viewable hologram in the form of many long thin slit holograms located side by side, thereby avoiding the need for creating an intermediate H1 hologram. Holographic printing schemes in which the final white-light-viewable hologram is printed directly without the need to generate an intermediate (H1) hologram are generally referred to as “1-step” or direct-write methods. Subsequent to this, a system was developed as described in U.S. Pat. No. 4,498,740 for the recording of two dimensional composite holograms composed of a two dimensional grid of separate holograms, each such hologram corresponding to a single object point. However, this latter system suffered from the disadvantage that the image should be located very close to the recording material. Additionally, the system was unable to form holograms which faithfully reconstructed the directional properties of light emanating from each image point.
U.S. Pat. No. 4,421,380 describes a system for producing 1-step full-colour transmission holograms from 3 interlaced strip or point composite holograms of the achromatic type by the inclusion of a registered colour-filter mask. U.S. Pat. No. 4,778,262 describes a 1-step method for writing directly a two dimensional matrix of basic holograms from computer data. Reference is also made to U.S. Pat. Nos. 4,969,700 and 5,793,503. U.S. Pat. No. 5,138,471 describes a similar technique whose preferred embodiment used a one dimensional spatial light modulator connected to a computer to directly write (1-step) common types of holograms as a two-dimensional matrix of basic holograms. U.S. Pat. No. 4,834,476 describes yet another similar 1-step technique based on computational or sequential camera data whose use was described for the direct writing of “Alcove” (curved) composite holograms having either a reflection or transmission geometry but which technique could be generalized to more conventional flat holograms.
Perhaps the most pertinent prior-art with regards 1-step direct-write holographic printers is the work of Yamagushi et al. (“Development of a prototype full-parallax holoprinter”, Proc. Soc. Photo-Opt Instrum. Eng (SPIE) vol. 2406, Practical Holography IX, pp 50-56 February 1995 and “High Quality recording of a full-parallax holographic stereogram with digital diffuser”, Optical Letters vol 19, no 2 pp 135-137 Jan. 20, 1994). This is discussed in more detail below and the known arrangement is described making reference to FIG. 16. A CW HeNe laser 1601 produces a beam which traverses an acousto-optic modulator 1602 before being relayed by mirrors 1603, 1604 and 1605 to the beam splitter 1609. The function of element 1602 is to act a simple shutter. At element 1609 the beam is broken into a reference arm and an object arm. The object beam passes through a [½] wave plate 1608 and a polarizer 1607 for polarization adjustment. It is then redirected by mirror 1606 before passing through telescope lenses 1612 and 1613. The beam is now steered by mirror 1614 to illuminate a twisted-nematic LCD panel 1615 having a resolution of 340×220 pixels with optional attached pseudorandom diffuser 1616 before being converged to a small spot of size 0.3 mm×0.3 mm on a photosensitive film 1620 within a defining aperture 1618 with a plunging mechanism 1619 for clamping said aperture and film together at each exposure.
The reference beam produced by element 1609 traverses the [½] waveplate 1610 and polarizer 1611 before being directed, via mirror 1621, onto the photosensitive substrate 1620 at the location defined by the aperture 1622, said aperture matching aperture 1618 but located on the reference beam side of the film.
The above system thus causes a reference beam and an object beam to co-illuminate a photosensitive film from opposite sides of said film in a small zone known as a holographic pixel or holopixel. The size of the holographic pixel thus made is effectively determined by the apertures 1618 and 1622. The object beam is focused down to said holographic pixel by the lens 1617 whose Fourier plane is arranged to lie on the photosensitive material 1620. By moving the photosensitive film 1620 in a two dimensional stepped manor and at each step changing the image in the LCD 1615, waiting for the system vibration to die out and then exposing a subsequent holopixel, a plurality of such holopixels are recorded onto the photosensitive film 1620. By computationally calculating all required LCD images a monochromatic white-light-reflection hologram is thus generated of a 3-D full parallax scene or object.
The above arrangement suffers from many disadvantages. Foremost the use of a CW laser severely limits the write time of each holographic pixel. In addition air currents, temperature changes and environmental sound will generally disturb the proper operation of such a printer. Hence, the arrangement suffers from a low printing speed, and it is not practically possible to implement such a device outside of a strictly controlled laboratory environment. It is to be noted, for example, that it is disclosed to take around 36 hours in order to write even a small hologram of 320×224 holopixels.
Another disadvantage of the above system is that it can only produce holographic pixels of one size. This is because both contact apertures 1618, 1622 and the fixed pseudorandom diffuser 1616 of pitch equal to that of the LCD are used to define the size of said holographic pixels. Both of these subsystems fundamentally constrain the holographic pixel size. Such a system is not therefore able to continuously change the holographic pixel size and hence different formats of holograms which require fundamentally different pixel sizes can not be readily produced.
The use of contact apertures 1618, 1622 in the system, apart from being inflexible, is also highly problematic since the emulsion surface of the photosensitive material is very sensitive.
Another disadvantage of this arrangement is that it is only designed to produce monochromatic reflection type holograms. Therefore transmission type holograms such as rainbows and achromats are precluded. The system is also unable to produce master H1 type holograms, and is similarly incapable of producing any form of multiple colour hologram.
Another disadvantage of the above system is that the wide-angle objective 1617 employed is designed to only minimize spherical aberration, is simplistic in design and only allows a restricted set of holographic formats to be produced.
Another disadvantage of the system is that the reference beam angle is fixed and cannot be controlled as may be required, for instance, to arrange for different hologram replay conditions. This is particularly problematic at large format.
As is readily apparent, the above described holographic printer suffers from numerous problems which render it impractical to use commercially.
In many cases the 2-step method of generating an intermediate H1 hologram from computer data and then copying or image-plane transferring this hologram to form a white-light viewable hologram (H2) is to be preferred over the above mentioned methods of directly writing the final hologram. This is due to a number of reasons. Firstly, it is frequently preferred to generate restricted parallax holograms, having only horizontal parallax. With the 2-step technique which produces an intermediate H1 hologram, such an H1 hologram may essentially be composed of one or more one-dimensional strips of overlapping holographic pixels. The classical optical transfer technique then takes care of the much harder computational step of calculating the distribution of light over the entire two-dimensional surface of the final (H2) hologram. If such a final hologram is written directly as in a 1-step printing scheme then this computation must be done by computer. In addition, for large holograms, the time required to write a two dimensional array of holographic pixels is usually proportional to the square of the time required to write the H1 master hologram and as such can become prohibitively long for some applications. Furthermore, a frequent complaint of directly written 1-step composite holograms is that the holograms appear “pixelated” whereas the 2-step technique of using an H1 master hologram is less prone to this problem.
Notwithstanding the above, there are many situations where it is advantageous to directly write the final hologram by a 1-step direct-write method. For example, directly written holograms are more easily tiled together to form ultra-large displays. Also in many applications quick previews of the final hologram are required and it is not generally convenient to produce an H1 hologram and then to put this hologram into another machine in order to generate the final H2 hologram. Additionally, the 1-step technique of directly writing holograms allows the creation of hybrid holograms having very non-standard viewing windows, something which is likely to be demanded by the printing industry in the context of holographic bill-board displays. Further advantages of the 1-step system are that materials such as photopolymers (see for example European patent EP0697631 B1) may be used which require only dry processing, whereas the more sensitive Silver Halide materials requiring wet processing must be employed for classically copied H2 holograms due to simple energy considerations.
Known 1-step and 2-step holographic printing processes employ CW lasers and thus, as a result, conventional holographic printing technology has been fundamentally slow and prone to vibrational disturbance.
In order to examine the salient features of the known 2-step holographic printers, the holoprinter described by U.S. Pat. No. 3,832,027 is reproduced in FIG. 15 and will be discussed below. A CW laser 41 emitting a monochromatic beam 71 is steered by prism 62 towards a beam splitter 43. Here the beam is divided into two parts. One part is commonly known as the reference beam and the other part as the object beam. The reference beam then further travels to a spatial filter and collimator (46 to 48) thus producing a collimated beam 72 which is steered by mirror 64 to an overhead tilted mirror 65 which finally directs said beam onto a photosensitive substrate 60 from above and at a suitable angle. A thin vertical aperture 58 covers the photosensitive substrate 60 in order to mask all but a thin vertical stripe 59 in said substrate.
The object beam emanating from optic 43 is reflected by prism 63 to a projection system 51 consisting of illumination lens 52, a photographic film transparency advance system 53 with film image 33 and a projection lens 54. The purpose of this projection system 51 is to project a magnified and focused image of the image, present on the film frame 33, onto the large diffusion screen 56 in coherent light. The light from this magnified image is then diffused by the diffuser in a wide variety of directions with some of said light falling onto the area of the photosensitive substrate 59 not covered by the aperture 58.
The system works by moving, in steps, the aperture across the photosensitive material surface in a direction orthogonal to the slit direction (ie vertically in the diagram and horizontally in reality) and by a finite amount, making a laser exposure at each such step. The film advance system is operated each time the aperture is moved such that the film image is changed at each exposure. By arranging that a set of appropriate perspective views of a certain 3-D scene or object are stored on the film roll, a holographic stereogram may thus be encoded on the photosensitive substrate 60.
There are many disadvantages of this system. Foremost, the use of a CW laser means that the entire system must be installed on a vibration isolation platform which must usually be pneumatically suspended. In addition air currents, temperature changes and environmental sound will generally disturb the proper operation of such a printer. Hence the system suffers from a low printing speed and it is impractical to use such a device outside of a strictly controlled laboratory environment.
Another disadvantage of this holoprinter is that a diffusion screen is utilized onto which 2-D perspective view images are projected. When the H1 hologram produced by this method is transferred to form an H2 hologram that is white-light viewable (see e.g. FIG. 6 of U.S. Pat. No. 3,832,027), the size of such final white-light viewable hologram (H2) must be less than or equal to the size of the diffusion screen 56. Thus, for example, if it is desired to generate a (1 m)×(1 m) white light viewable hologram then a diffusion screen of at least (1 m)×(1 m) size must be used. Since the distance D shown in FIG. 15 must correspond to both the final optimum viewing distance of the white-light-viewable hologram and the distance D, shown in FIG. 6 of U.S. Pat. No. 3,832,027, such distance D must usually be rather greater than the hologram size. One can thus see that the intensity of object light finally falling through the slit 59 of the aperture 58 onto the photosensitive material 60 of FIG. 15 is many orders of magnitude less than the total light illuminating the diffusion screen. In the case that it is desired to generate a white-light-viewable hologram (H2) of size (1 m)×(1 m) by the process described in FIG. 6 of U.S. Pat. No. 3,832,027, a value of D shown in FIG. 15 of approximately 1 m may sensibly be adopted. Taking the average sensitivity of standard Silver Halide holographic film to be 50 μJ/cm2 and making various realistic system approximations it can be shown that a minimum laser energy of 1 Joule is required. Therefore, in order to write such holograms, either a large CW laser would be required or very long exposures must be used. However a powerful laser is undesirable due to the problems of thermal heating of the various optical components, particularly the film 33, which must remain interferometrically static during each and every exposure. Long exposure times are undesirable because of problems due to vibration.
Another disadvantage of the above system is that a diffusion screen, aside from being energetically inefficient, inevitably deteriorates the image quality.
Another disadvantage of the above system is that a point source is used to illuminate the film transparency and thus the final image fidelity will be severely limited.
Another disadvantage of the above system is that a large moving aperture must move in quasi-contact with the photosensitive emulsion surface. This is usually very problematic as the emulsion of the photosensitive material 60 is usually highly fragile and yet if the aperture 58 is held at more than a very small distance from said emulsion surface then the quality of the generated hologram will rapidly fall.
A yet further disadvantage of the above arrangement is that the moving aperture will inevitably leave areas of the hologram which are either doubly exposed or unexposed, thus diminishing the quality. This is particularly true when the slit size 59 is much smaller than the hologram size.
Another disadvantage of the above arrangement is that it is only capable of making H1 type holograms and cannot directly write 1-step white-light-viewable holograms where the 3-D object bisects the hologram plane.
A further disadvantage of the above arrangement is that it is only capable of reasonably writing single parallax holograms as generalization of the technique to full parallax would render the technique hopelessly cumbersome given the above cited problems. A commercial holographic printing device must be expected to be relatively compact, operate in a normal commercial environment which is prone to vibrations, produce a variety of hologram formats and possess reasonable print times.
Accordingly it is desired to provide an improved holographic printer.